This invention relates to a keyboard switch suitable for use with various kinds of equipment such as personal computers, word processors, and the like, and particularly to a keyboard switch having keytops configured to be supported by means of pantograph mechanisms.
FIG. 1 illustrates an example of the heretofore proposed keyboard switch having such configuration in which on a plate 11 made of a metal plate, for example there is disposed a membrane sheet (contact sheet) 12 on which a base 13 made of a metal plate is positioned. The membrane sheet (contact sheet) 12 comprises,a pair of laminated contact layers 12B and 12C spaced apart by a spacer 12A, and a movable contact pattern and a pair of fixed contact patterns (not shown) formed on the opposed surfaces of the contact layers 12B and 12C, respectively, the arrangement being such that the contact layer 12B is adapted, when depressed toward the contact layer 12C, to bring the movable contact pattern into contact with the pair of fixed contact patterns. It is to be noted that there are a plurality of the sets of the movable contact pattern and the fixed contact patterns are provided so that any selected one of the sets may be actuated to make contact.
As shown in FIG. 2, the base 13 has generally square openings 13A formed therethrough, each for one of the sets of contact patterns, and is formed with a pair of first journal bearing portions 13B and a pair of second journal bearing portions 13C projecting oppositely from the membrane sheet 12 along the two opposed side edges of each opening 13A. The pair of first journal bearing portions 13B are opposed to each other and are formed in the form of a protrusion having a generally trapezoidal contour in cross-section when the base 13 is formed by a drawing process, for example while the pair of second opposed journal bearing portions 13C are formed in the form of a protrusion having a generally, semi-circular contour in cross-section when the base 13 is formed by a drawing process (FIG. 1).
A pair of links 14 and 15 comprising a pantograph mechanism 24 is disposed on the membrane sheet 12 for each of the sets of the contact patterns with the pair of links 14 and 15 supported at their one ends in the associated journal bearing portions 13B and 13C, respectively.
As shown in FIGS. 3A and 3B, the first link 14 is generally U-shaped and comprises a shaft 14A and a pair of legs 14B extending from the shaft at the opposite ends thereof and having stud shafts 14C protruding outwardly therefrom at their forward ends and away from each other. The legs 14B further have columnar (solid cylindrical) bosses 14D protruding inwardly toward each other from the inside surfaces thereof intermediate their opposite ends. In addition, the legs 14B have stops 14E protruding inwardly toward each other from the inside surfaces thereof adjacent their forward ends.
The second link 15, as shown in FIGS. 4A and 4B, comprises a generally annular frame-like portion 15A having a pair of spaced apart extensions 15B extending in the same direction from the outer periphery of the frame-like portion along one semi-circular segment (forward end side) thereof. The two extensions 15B have stud shafts 15C extending inwardly toward each other therefrom adjacent the forward ends thereof. Further, a pair of stud shafts 15D extend outwardly from the base end portion (the other semi-circular segment) of the frame-like portion 15A in parallel to the stud shafts 15C. The frame-like portion 15A is formed in the diametrically opposed outside surfaces in the center thereof with shaft receiving holes 15E, respectively. The first and second links 14, 15 are typically made as molded parts of synthetic resin such as polyacetal (POM).
As shown in FIG. 2, the links 14 and 15 are configured such that the frame-like portion 15A of the link 15 is accommodated between the opposed legs 14B of the link 14 with the inside surfaces of the legs 14B facing the outside surfaces of the frame-like portion 15A and with the pair of bosses 14D matingly engaged in the corresponding holes 15E of the link 15 whereby the two links 14 and 15 are assembled and connected together for relative pivotal movement. It should be noted here that the stops 14E provided on the link 14 serve to limit the relative movement of the links 14 and 15 toward the overlapping relation beyond a predetermined position.
Prior to superposing the membrane sheet 12 over the base 13, the pair of stud shafts 14C of the link 14 are inserted in the corresponding journal bearing portions 13B of the base 13 from the membrane sheet 12 side to be rotatably and movably (slidably) supported therein, while the pair of stud shafts 15D of the link 15 are likewise inserted in the corresponding journal bearing portions 13C of the base 13 to be rotatably supported therein.
On the other hand, the shaft 14A of the link 14 is rotatably fitted in a pair of journal bearings 16A formed in the bottom side of the keytop 16 while the pair of stud shafts 15C of the link 15 are rotatably and slidably supported in corresponding slide bearings 16B formed in the bottom side of the keytop 16.
A dome-like rubber member 17 is mounted on the membrane sheet 12 within the opening 13A in the base 13 and extends through the central aperture of the frame-like portion 15A so as to be is interposed between the keytop 16 and the membrane sheet 12. The rubber member 17 comprises a cylindrical portion 17A fitted over a projection 16C extending from the keytop 16, and a dome portion 17B connecting with the cylindrical portion. The dome portion 17B has a push-button portion 17C projecting inwardly from the ceiling thereof for pressing down on the corresponding contact region of the membrane sheet 12.
With the keyboard switch constructed as described above, downward pressing operation on the keytop 16 will move the keytop 16 generally parallel to the face of the base 13 by means of the links 14 and 15 in the form of the pantograph mechanism while the keytop 16 presses on the rubber member 17 so that the dome portion 17B is deformed and collapsed down. Through this process, the user will get a good tactile feeling while at the same time the contact region of the membrane sheet 12 is pressed on by the push-button portion 17C whereby the electrical continuity is established. On the contrary, upon the downward pressure on the keytop being released, the rubber member 17 returns to its original position by its resilient restoring force to cut off the electrical continuity through the contact region whereby the keytop 16 is restored to its original position.
It should be noted here that an increasing reduction in the thickness of the keyboard switch of the type discussed herein is accompanied with a reduction in component space allowed to be allotted for the pantograph mechanism (linkage), inevitably leading to an increasing demand for reduction in both size and vertical profile (thickness) of the pantograph mechanism. Such a situation has resulted in a decrease in the rigidity of the linkage, and hence a greater vulnerability of the linkage to deformation.
More specifically, comparison between the links 14 and 15 has shown that the U-shaped link 14 is more vulnerable to deformation than the link 15 comprising the frame-like portion 15A as a central body portion. Moreover, there is more freedom of motion involved in the support of the stud shafts 14C provided at the forward ends of the legs 14B of the link 14 by the journal bearing portions 13B of the base 13 than in the support of the stud shafts 15D of the link 15 by the journal bearing portions 13C of the base 13. For this reason, when a force to pull the keytop 16 upward from its unactuated state is applied to the keytop as in an attempt to remove dust, for instance, such a situation may occur that the link 14 is pulled up together with the keytop 16 so that the forward ends of the legs 14B are deformed inwardly toward each other as shown by arrows 18 in FIG. 2 with the result that the stud shafts 14C are dislodged from the journal bearing portions 13B, and hence the link 14 is detached from the base 13.
Should such situation occur, common users using such keyboard would be unable to repair it by themselves, so that they would have to ask servicemen for repairs. In this regard, occurrence of link detachment has been hitherto a serious trouble.
Accordingly, an object of this invention is to provide a keyboard switch configured such that even a force applied on a keytop tending to pull it up may not possibly uncouple the associated link from the base.
According to this invention, a keyboard switch is provided which comprises a pantograph mechanism comprising a U-shaped first link having opposed legs and a second link including a frame-like portion embraced between the opposed legs of the first link, the opposed legs having first stud shafts protruding outwardly therefrom at their forward ends and away from each other, the first stud shafts being rotatably and movably supported in first bearing portions formed. on the base, the frame-like portion of the second link having second stud shafts extending outwardly therefrom at the base end portion thereof, the second stud shafts being inserted and rotatably supported in second bearing portions formed on the base, one of the inside surfaces of the opposed legs and the opposed outside surfaces of the frame-like portion opposing the corresponding inside surfaces of the legs having a pair of bosses protruding therefrom, the bosses being engaged in a pair of shaft receiving holes formed in the other of the inside surfaces of the opposed legs and the opposed outside surfaces of the frame-like portion, respectively to form the pantograph mechanism; a keytop supported by the first and second links; a membrane sheet (contact sheet) disposed on the side of the base opposite from the keyboard and having contact patterns formed thereon; and an actuating member operative in response to vertically downward and upward movements of the keytop relative to the membrane sheet to turn the contact patterns on and off, wherein projection means is formed on one of the inner sides of the opposed legs and the opposed outer sides of the frame-like portion and depression means for accommodating the projection means is formed in the other of the inner sides of the opposed legs and the opposed outer sides of the frame-like portion, one of the projection means and the depression means being formed with tapered surfaces, the arrangement being such that with the keytop in its unactuated position, the tapered surfaces on the one of the projection means and the depression means and angular edges of the other of projection means and the depression means are in abutment with or close to each other, and the tapered surfaces and the angular edges are forced into contact with each other when the keytop is pulled up from its unactuated position away from the membrane sheet, which urging force in turn urges the opposed legs to move such that their forward ends are forcedly expanded apart from each other.